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The rapid release of methane into the earth's atmosphere 635 million years ago caused runaway global warming, and may happen again in the near future, a new study reports.

The study, which appears in this week's edition of Nature, provides an insight into what may happen to the earth's atmosphere if today's frozen methane deposits in Canada, Siberia and Alaska begin to thaw.

During the Cryogenian period (850-630 million years), earth was completely frozen over, with glaciers that crept down into the tropics and possibly even reached the equator.

Also known as the 'snowball earth' era, the planet's chill was self-sustaining.

The ice covering the planet forms a brilliant white shell that reflected the sun's rays, preventing the surface from warming.

But after 155 million years of being in a deep freeze the earth warmed dramatically.

How this happened has been fiercely disputed, although all agree that the event changed the planet's climate system and ocean chemistry forever.

Scientists now point the finger at methane clathrates. These methane-rich ice deposits form under ice sheets at specific temperatures and pressures.

Vicious cycle

The study's authors, who included Australian geologist Emeritus Professor Chris von der Borch from Flinders University, believe that at the end of the Cryogeian period, the ice sheets covering the earth became unstable, which released pressure on the clathrates.

As the clathrates began to evaporate, they released methane, which helped trap more solar heat and warmed the planet.

This thawed more clathrates, fuelling further warming and so on, creating a vicious circle.

Methane is considered a prodigious greenhouse gas, being 30 times more efficient than CO2 in trapping solar heat.

Martin Kennedy, a geologist at the University of California Riverside who led the study, says evidence of rapid thawing comes from hundreds of marine sediment samples taken in South Australia.

Analysis of the oxygen isotopes in the sediments gave a signature of melting waters in ice sheets and destabilisation of clathrates by the meltwater.

Kennedy says the findings have a bearing on a much-feared positive feedback today.

Arctic concerns

"One way to look at the present human influence on global warming is that we are conducting a global-scale experiment with earth's climate system," says Kennedy.

"We are witnessing an unprecedented rate of warming, with little or no knowledge of what instabilities lurk in the climate system and how they can influence life on earth."

The thawing of clathrates frozen in the Arctic polar region could release billions of tonnes of methane into the atmosphere.

The concern is that it could take a relative small rise in temperature to start unleashing the gas, which would then trigger an unstoppable warming cycle.

If the end of snowball earth is a guide, positive feedbacks, "once initiated, change the climate to a wholly different state," he says.